IMO code/service owners. Orgs can be scaled by splitting code ownership into teams owning different services. And if someone isn’t competent enough to ship code on the service they’ve been assigned to work on, they aren’t qualified to be employed working in that thing. They aren’t providing value to the organization (probably net negative).

PRs don’t actually solve the “but what if someone ships crap” problem. Taking capacity from competent engineers to review and correct other employee’s code defeats the purpose of employing people: to add capacity.

Juniors/employees in training should be shadowing or paired with someone who is competent until they are competent, and assigned work at their level.